Ray Nicanor M. Tag-at, Lloyd Henry Li

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1 A Study on the Application of On-Chip EOS/ESD Full-Protection Device for TMR HeadsRay Nicanor M. Tag-at, Lloyd Henry LiHitachi Global Storage Technologies, Phil. Corp.Application of on-chip ESD protection for TMR heads has already been studied for many years. As the TMR technology advances and becoming more ESD sensitive, it is important to continuously understand these protection devices and assess it’s applicability with the current TMR technology. Good Morning! I’m Ray Tag-at of Hitachi GST Phils., and I’ll be presenting our technical paper entitled “A Study on the Application of On-Chip EOS/ESD Full-Protection Device for TMR Heads.”

2 ObjectivesTo study and understand the different ESD protection devices for TMR Heads.To have an effective ESD protection devices that could enhance the robustness of the TMR heads.We came up with this paper to deeply understand the different protection devices for TMR heads, their application, their advantages and disadvantages…and be able to come up with an effective protection device or method that could enhance the ESD robustness of the TMR heads.

5 IntroductionShunting is a commonly used method for on-chip ESD protection.Diode can also work as a shunt across the TMR sensor.Commonly installed across tester’s TMR input terminals.Protects from electrical transients and EOS.There are already many published studies about diode shunting in GMR/TMR heads.Shunting is a commonly-used method for on-chip ESD protection, and gold wire is usually used. The diode can also work as a shunting device, where is it usually installed across the tester’s TMR terminals. This protects the device for hot-plugging transients and electrical overtsress. And this method has already been extensively studied.Slide 5

6 Introduction What are the downsides of this method?Diodes can also be charged up through its capacitance.TMR heads has no protection on the rest of the assembly process.Is it then possible to install shunt diodes into the device itself?But what are the downsides of this method? The diodes, having a capacitance, can be charged up from the tester’s input signal and this can still cause a machine model ESD event. Also, since the diodes are only installed in the testers, the TMR has no protection on the rest of the assembly process. By having these concerns, is it then possible to install the shunt diodes into the device itself?Slide 6

9 Diode’s CharacteristicP-N semiconductor diode has insulating region so-called the “depletion region.”p- dopedn-dopedDepletion RegionDepletion Region dictates the flow of current.It becomes conductive in Forward Bias and remains insulative at Reverse Bias.Let us first understand the characteristics of a diode. Most diodes have insulating region called the depletion region that dictates the flow of current. This region becomes conductive at forward bias and remains insulative at reverse bias. This ability of the diode makes it ideal to be used as shunting device.Diode’s capability to become conductive means that it can be used as shunting device.Slide 9

11 Diode’s Characteristic CurvesI-V CurvesR-V CurvesSelection of diode is important. For this study, the diode used in D1n From the I-V curve, this diode turns on at around 300 mV, with On resistance of around 300 ohms from the R-V curve.D1N5711 diode “Turn-on” voltage: @ 0.3 VDiode “on” Resistance, 300 ohmsTMR Head’s nominal resistance: 500 ohmsSlide 11

12 Shunt Diode’s BehaviorDuring Normal Testing and OperationPSPICE SimulationTypical test and operating signal of the TMR head is around 100 to 150 mV. To see whether the diodes can affect this signal or not, we run the PSPICE simulation. Results shows that at lower frequency, around 1 MHz to 100 MHz, the operating signal across the TMR is the same on with and without shunt diodes. At higher frequency, around 4GHz, the TMR operating sinal losses about 3% of it’s value when diodes are installed. Considering some tolerances, this difference is not that significant.At lower operating frequency, the TMR input signal is the same with and without diode.At higher frequency, the voltage across the TMR losses 3% of it’s operating voltage when diodes are installed.Slide 12

14 Shunt Diode’s BehaviorResults:HGA DET Test SignalNo DiodeWith DiodePeak Voltage: 122 mVPeak Voltage: 122 mVResult from the DET tester shows that the peak voltages and frequency on with and without diodes are the same. The test parameters from the HGA Quasi tester are also the same on with and without diodes.HGA Quasi Test ParametersNo difference in the Test Signal on with and without shunt diodes!Slide 14

15 Shunt Diode’s BehaviorDuring EOS/ESD EventsPSPICE SimulationAfter knowing that the diodes will not affect the TMR’s operating signal, we then evaluated it’s behavior in an ESD event. This is the PSPICE simulated circuit for MM and HBM ESD events, the TMR and the diodes.This simulates a HBM and MM ESD events during testing or handling during fabrication and assembly.Slide 15

16 Shunt Diode’s BehaviorPSPICE Simulation Results:Result shows that as the ESD Voltage increases, the transient current across the TMR is lesser when shunt diodes are installed. This means that the diodes can increase the ESD threshold of the TMR.MM and HBM ESD threshold of the TMR head increases when shunt diodes are applied.Slide 16

17 Shunt Diode’s BehaviorESD Testing of TMR HeadsWe verified this result using a machine model ESD simulator, actual diodes, and TMR heads. Shown here are the failure thresholds of the TMR head. The failure threshold of the TMR heads with shunt diodes are higher than those without.Shunt diodes can indeed increase the ESD threshold of the TMR head.Slide 17

18 Shunt Diode’s BehaviorWhat are the downsides of this method?The device has no protection from various ESD events throughout the entire fabrication and assembly.Only provides protection from Tester’s transients and electrical overstress (EOS).The diodes can still be charged up when installed in the Tester’s preamplifier.We know that diodes can offer protection to the TMR heads, however there are some downsides in the conventional shunting method. First, since diodes are installed only in testers, the TMR heads have no protection throughout the fab and assembly process, and that the diodes only provides protection from the tester’s hot-plugging transients and electrical overstress. And since diodes have capacitance, it can still be charged.Slide 18

19 Charging Mechanism of DiodesDiodes have diffusion capacitance (Cd) and zero p-n junction capacitance (Cj).The charge stored in the neutral regions adjacent to the junction.The amount of charge stored is proportional to the forward current.Proportionality constant is called Transit Time (TT).How diode is charged? Diodes have diffusion capacitance and zero p-n junction capacitance. The junctions are like parallel plates separated by an insulator can the depletion region, where the charge is being stored. As shown in this equation, the amount of charge us proportional to the forward current, with constant called the transit time.Slide 19

20 Charging Mechanism of DiodesNonlinear charging mechanism leads to a nonlinear capacitance.This nonlinear charging mechanism leads to a nonlinear capacitance. As shown in this chart, as the forward bias voltage increases, the capacitance also increases. From Q=CV, the charge would then increase.From Q = CV:Increase in capacitance would increase the charge, Q.Slide 20

21 Charging Mechanism of DiodesEffect of the Charge Storage MechanismPSPICE Simulation ModelTo evaluate this theory, we again use the PSPICE for ESD event simulation. In here, the diodes where simulated into a model that shows the capacitance, where it can be charged up and cause ESD transients into the TMR.This simulates an ESD event from the tester with charged diode.Slide 21

22 Charging Mechanism of DiodesEffect of the Charge Storage MechanismPSPICE Simulation ResultsResult shows that the ESD Transient Current at the TMR increases with the increase of the charge of the diodes.Transient current in the TMR sensor increases with the increase of charge at the diodes.Slide 22

23 Charging Mechanism of DiodesEffect of the Charge Storage MechanismActual ESD Transient MeasurementTEK CT-6 current probe with 200-ohm simulated TMR resistance was used.Tap transient test was done at the HGA QST and DET Tester’s TMR input pins.We then verified this theory using the HGA QST ad DET Testers where the diodes are installed. Right after each test, we performed tap transient using CT-probe with simulated TMR resistance at the tester’s probe pins.Slide 23

24 Charging Mechanism of DiodesEffect of the Charge Storage MechanismActual ESD Transient Measurement ResultsResult shows that indeed, transient current can still be detected at the tester’s probe pins. The transient waveform signature is similar to that of the machine model ESD event.A Machine Model (MM) ESD event waveform was detected at the Tester’s probe pins with shunt diodes!Slide 24

25 On-Chip Diode ShuntingConcept similar to the typical diode shunting.This method proposed that the diodes will be placed into the TMR head itself.Typical Diode ShuntingOn-chip Diode ShuntingWith this problem in mind, we came up with concept called on-chip diode shunting. The concept is still the same with the typical diode shunting, however in this method, the diodes will be made small enough and be placed into the TMR head itself.Shunt diodes somehow does not affect the operating signal, thus it installed into the TMR Head itself!Slide 25

26 On-Chip Diode ShuntingProtection Capability ComparisonNo ShuntTypical Diode ShuntingOn-chip Diode ShuntingElectrical Overstress (EOS)XOESDHere’s the summary of the protection capability of the typical method of diode shunting and the on-chip diode shunting. The typical diode shunting only provides protection from testers hot-plugging transients and electrical overstress, but not on ESD from various sources throughout the entire fab and assembly process, while the on-chip shunting method offers protection for both.X – cannot protectO – can protectSlide 26

27 ConclusionsDiode shunting can increase the TMR head’s threshold from EOS/ESD Events.If installed in the tester’s preamplifier, diodes can still be charged up, causing ESD transients.Properly selected diodes can be installed into the device as on-chip ESD protection.In summary, the diodes can be used as shunting device and can increase the TMR head’s threshold from EOS/ESD Events. However, one should be take note that if diode installed in the tester’s preamplifier, it can still be charged up through its capacitance, causing ESD transients. Also, properly selected diodes can be installed into the device as on-chip ESD protection, which can provide protection all throughout the TMR head assembly process.Diodes installed as on-chip ESD protection offers protection all throughout the TMR head assembly process.